Abstract
BackgroundThe defect structure of organic materials is important as it plays a major role in their crystal growth properties. It also can play a subcritical role in “hot-spot” detonation processes of energetics and one such energetic is cyclotetramethylene-tetranitramine, in the commonly used beta form (β-HMX).ResultsThe as-grown crystals grown by evaporation from acetone show prismatic, tabular and columnar habits, all with {011}, {110}, (010) and (101) faces. Etching on (010) surfaces revealed three different types of etch pits, two of which could be identified with either pure screw or pure edge dislocations, the third is shown to be an artifact of the twinning process that this material undergoes. Examination of the {011} and {110} surfaces show only one type of etch pit on each surface; however their natural asymmetry precludes the easy identification of their Burgers vector or dislocation type. Etching of cleaved {011} surfaces demonstrates that the etch pits can be associated with line dislocations. All dislocations appear randomly on the crystal surfaces and do not form alignments characteristic of mechanical deformation by dislocation slip.ConclusionsCrystals of β-HMX grown from acetone show good morphological agreement with that predicted by modelling, with three distinct crystal habits observed depending upon the supersaturation of the growth solution. Prismatic habit was favoured at low supersaturation, while tabular and columnar crystals were predominant at higher super saturations. The twin plane in β-HMX was identified as a (101) reflection plane. The low plasticity of β-HMX is shown by the lack of etch pit alignments corresponding to mechanically induced dislocation arrays. On untwinned {010} faces, two types of dislocations exist, pure edge dislocations with b = [010] and pure screw dislocations with b = [010]. On twinned (010) faces, a third dislocation type exists and it is proposed that these pits are associated with pure screw dislocations with b = [010].Graphical abstractEtch pits on the twinned (010) face of β-HMX.
Highlights
Understanding the crystal growth, morphology and defect structure of organic materials has become increasingly important as the breadth of their commercial usage increases
Crystal growth The crystals obtained by the growth methods employed in this study exhibited only the forms {011}, {110}, (010) and (101) irrespective of growth conditions
Crystals of β-HMX were grown from acetone solution by solvent evaporation of spontaneously nucleated solutions and by solvent evaporation and slow cooling of seeded solutions
Summary
Understanding the crystal growth, morphology and defect structure of organic materials has become increasingly important as the breadth of their commercial usage increases. HMX occurs in four polymorphic forms, α, β, δ and ε [1,2,3,4,5,6] (and a hydrated form, termed γ [7]), of these the most stable thermodynamically is β-HMX This form is monoclinic, space group P21/n, a0 = 6.526 Å, b0 = 11.037 Å, c0 = 7.364 Å, β = 102.66°. The defect structure of organic materials is important as it plays a major role in their crystal growth properties. It can play a subcritical role in “hot-spot” detonation processes of energetics and one such energetic is cyclotetramethylene-tetranitramine, in the commonly used beta form (β-HMX)
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